1. Field of the Invention
The present invention is directed to a cable guide for a nuclear magnetic resonance tomography apparatus of the type having a magnet with an opening serving as the examination space with an inside wall on which a whole-body antenna, azimuthally divided into a plurality of sub-antennas, is attached, and into which a patient bed can be introduced.
2. Description of the Prior Art
There is often a need in nuclear magnetic resonance tomography devices to conduct cables from devices arranged at the patient bed to devices disposed outside of the examination space. Devices known as local coils, for example, are often used and are attached as close as possible to the examination region in which the patient is disposed. The local coils are adapted in shape and size to the body part to be imaged. The radio frequency pulses needed to excite the nuclear spins are transmitted by an antenna known as a whole-body antenna, and reception of the nuclear magnetic resonance signals ensues with the local coil. A better signal-to-noise ratio is achieved because the reception region is limited to the examination region of interest. The local coils are secured at the patient bed, or to the patient, while the bed is entered into and withdrawn from the examination space.
In standard nuclear magnetic resonance tomography devices, the nuclear magnetic resonance signals received by the local coil are supplied via a RF cable to a pre-amplifier located outside of the magnet which generates the fundamental field. The local coil, by contrast, is situated within the whole-body antenna, preferably in the immediate proximity of the antenna center because the field homogeneity is best at that location.
In addition to the RF cable for the local coil, further lines to the patient bed are needed in many instances, for example for transmitting control signals for the local coil and for transmitting ECG and respiratory signals of the patient for triggering the imaging.
Problems occur due to cables which must be conducted out of the center of the whole-body antenna when the patient bed is introduced into the opening in the center of the magnet. High electromagnetic fields are present in the whole-body antenna during the transmission phase. Coupling between the cables and the antenna, and thus an RF power flow in the exterior of the cable shielding arise due to skin-effect waves. This can cause considerable disturbances both in the picture quality and in the electronic components. Coupling of the antenna field to the lines ensues magnetically (inductively) and electrically (capacitively).
The magnetic coupling can be minimized by avoiding the formation of loops by the lines inside the whole-body antenna. The electrical coupling can be minimized by conducting the cables to the exterior from the center of the whole-body antenna in a defined fashion in zones of low electrical field strength of the whole-body antenna.
In systems using a linearly polarized antenna, such zones are found in the central cross-sectional plane of the antenna, as well as in the proximity of the antenna shielding foil centrally between the antenna halves.
In systems using a circularly polarized antenna, zones of low electrical field strength are also found in the central cross-sectional plane of the antenna.
In known systems, this zone is used for cable guidance by disposing an electrically shielding cable channel at that location in the form of a metallic profile having a U-shaped cross-section. In such known arrangements, the cable channel thus is disposed above the patient bed. Connection between the cable channel and the patient bed is produced using carriers secured to the patient bed. These carriers, however, cause certain restrictions in the bearing comfort of the patient, because they partially restrict the examination space, which is already designed as tightly as possible for reasons of economic feasibility and RF fields.
German Gebrauchsmuster 84 13 651 discloses a patient bed having a plug mechanism for the connection of a cable to a signal transmission means. This cable is rigidly secured to the patient bed and leads to a stationary apparatus part via a loop in order to enable longitudinal displacement of the patient bed.
German Gebrauchsmuster 89 09 245 also discloses cables leading from a stationary apparatus part to plug mechanisms, which are provided at a side of a patient support table. These cables are conducted along the side of the patient support table, and are surrounded by a cable guide formed by an articulated cladding having a rectangular cross section.
German OS 33 27 731 discloses means for acquiring an ECG signal in a nuclear magnetic resonance tomography apparatus. The connecting cable between the ECG electrodes and the ECG processing means includes cable shielding electrically connected so as to be at the potential of the RF shielding of the tomography apparatus.